Crucible for melting rare earth element alloy and rare earth element alloy

ABSTRACT

A crucible comprising Al 2 O 3  and at least one selected from rare earth oxides inclusive of Y 2 O 3  as main components and characterized by firing at 500-1,800° C., the distribution of the rare earth oxide at a higher proportion in a fine particle portion having a particle size of-up to 0.5 mm than in a coarse particle portion having a particle size in excess of 0.5 mm, and the substantial absence of the reaction product of the rare earth oxide with Al 2 O 3  is suitable for the melting of a rare earth alloy.

TECHNICAL FIELD

This invention relates to a crucible for use in the high-frequencymelting of rare earth alloys, and rare earth alloys obtained using thecrucible.

BACKGROUND ART

Rare earth alloys are recently used in a variety of fields as magnetmaterial, battery electrode material and the like. It is important tomanufacture rare earth alloys of consistent quality at a low cost.

In general, the rare earth alloys are produced by weighing amounts ofraw materials so as to give a desired composition, placing them in acrucible, high-frequency melting, and feeding the melt into a mold orrotating rolls for casting.

The crucible used for high-frequency melting can be manufactured by aconventional process, as used in the ceramic preparation, involvingmixing predetermined raw materials with water or a binder and water toform a slurry, and shaping the slurry, followed by removal from the moldand firing (or drying). Since the molten rare earth alloy isextraordinarily reactive, the crucible used for melting of the cruciblematerial must meet the following requirements.

Namely, the crucible material must have (1) least reactivity with waterand (2) a certain degree of stability. On the other hand, the crucibleis given requirements including (1) resistance to cracking by thermalexpansion upon high-frequency heating (resistance to thermal shocks),(2) high mechanical strength, (3) easy removal of the slag whichdeposits within the crucible at the end of melting, and (4) low cost.

To meet these requirements, the crucibles used in the melting of rareearth alloys have been constructed of Al₂O₃ or Al₂O₃ with additives.

Al₂O₃ used herein has a stability which is relatively high among oxides,but inferior to rare earth metals, allowing reaction to proceed littleby little. The reacted portion becomes a slag. The slag, which isstrongly bound to the crucible and difficult to remove, gives rise tosome problems that the slag removal operation takes a long time todetract from productivity and that the crucible can be damaged duringthe slag removal operation.

These problems may be solved by using rare earth oxides and calciumoxide which are highly stable and least reactive with rare earth metals.Because of their likelihood to form hydroxides, they can be used asunshaped refractory or added only in small amounts, but are difficult touse as the primary material of shaped refractory to construct thecrucible.

DISCLOSURE OF THE INVENTION

Therefore, an object of the invention is to provide a rare earth alloymelting crucible which allows any slag generating thereon to be readilyremoved, can be used repeatedly, and is inexpensive; and a rare earthalloy obtained using the crucible.

Analyzing the reaction of a conventional crucible made of Al₂O₃ with anNd—Fe—B base alloy typical of rare earth alloys and a slag thereof, theinventors found the following problems.

More particularly, since the particle size formulation of the crucibleis important in order that the crucible be unsusceptible to cracking bythermal expansion upon high-frequency heating (or be improved instrength to thermal shocks), the particle size distribution is adjustedin accordance with the purpose or the size of the crucible. Al₂O₃particles having a particle size of up to 2 mm, especially up to 0.5 mmpreferentially undergo reaction with Nd or rare earth metal as shown bythe equation:Nd+Al₂O₃→Al+Nd₂O₃which reaction proceeds progressively; whereas Al₂O₃ particles having aparticle size of more than 2 mm undergo reaction to a depth of 10 to 100μm from their surface, but the reaction does not proceed further as ifthe particles were protected by the reacted film. This difference arisespresumably because finer powder becomes more active.

The progress of such reaction results in a state as shown in FIG. 2.When the reaction reaches the interior of a crucible 1 composed ofcoarse particle portions 10 and fine particle portions 11, the regioinof the crucible that has reacted forms a slag 1A which is strongly boundto the crucible 1. When melting of a subsequent batch is carried out inthis crucible with the slag 1A left intact, the reaction proceedsfurther and the internal volume of the crucible varies. It is thenbasically required to remove the slag 1A every batch or periodically.Since the slag 1A is strongly bound to the crucible 1, removal operationis difficult and can damage the crucible 1 during the procedure.

Then making studies from several aspects, the inventors have found thatthe life of the crucible can be increased by introducing a rare earthoxide into portions with a particle size of up to 0.5 mm at a highconcentration to prevent the crucible material from reacting with a rareearth alloy, whereby the slag if generated can be readily removed.

On the other hand, since melting of a rare earth alloy is carried outbatchwise, thermal cycles of heating from room temperature to a meltingtemperature (1,000 to 1,700° C.) and then cooling are repeated. Thecrucible is then subjected to repeated cycles of thermal expansion andcontraction. This can cause the crucible to be cracked, resulting infurther promoted degradation.

Referring to FIG. 3, the phenomenon is described. The above-describedthermal cycles cause the crucible 1 composed of coarse particle portions10 and fine particle portions 11 to crack at 1B. As a result, reactionreaches the interior of the crucible 1, and the reacted region forms aslag 1A which is strongly bound to the crucible 1. When melting of asubsequent batch is carried out in this crucible with the slag 1A leftintact, the reaction proceeds further and the internal volume of thecrucible 1 varies. It is then basically required to remove the slag 1Aevery batch or periodically. However, since the slag 1A is stronglybound to the crucible 1, removal operation is difficult and can damagethe crucible 1 during the procedure. In this connection, it has beenfound that generation of cracks on thermal cycles can be prevented byusing Al₂TiO₅ having a low coefficient of thermal expansion as a maincomponent, and that the life of the crucible can be increased byintroducing a rare earth oxide into portions with a particle size of upto 0.5 mm at a high concentration to prevent the crucible material fromreacting with a rare earth alloy, whereby the slag if generated can bereadily removed. The present invention is predicated on these findings.

Specifically, the present invention provides:

(1) a crucible for the melting of a rare earth alloy, comprising Al₂O₃and at least one selected from rare earth oxides inclusive of Y₂O₃ asmain components, characterized in that the crucible is obtained byfiring at 500 to 1,800° C., the rare earth oxide is distributed at ahigher proportion in a portion of fine particles having a particle sizeof up to 0.5 mm than in a portion of coarse particles having a particlesize in excess of 0.5 mm, and the crucible is substantially free of thereaction product of the rare earth oxide with Al₂O₃;

(2) a crucible for the melting of a rare earth alloy according to (1),characterized in that 2 to 100% by volume of the fine particle portionis the rare earth oxide and 20 to 100% by volume of the coarse particleportion is Al₂O₃; and

(3) a rare earth alloy obtained using a crucible according to (1) or(2).

Also, the present invention provides:

(4) a crucible for the melting of a rare earth alloy, comprising Al₂TiO₅and at least one selected from rare earth oxides inclusive of Y₂O₃ asmain components, characterized in that the crucible is obtained byfiring at 1,000 to 1,700° C., the rare earth oxide is distributed at ahigher proportion in a portion of fine particles having a particle sizeof up to 0.5 mm than in a portion of coarse particles having a particlesize in excess of 0.5 mm, and the crucible is substantially free of thereaction product of the rare earth oxide with Al₂TiO₅;

(5) a crucible for the melting of a rare earth alloy, comprising Al₂O₃,Al₂TiO₅ and at least one selected from rare earth oxides inclusive ofY₂O₃ as main components, characterized in that the crucible is obtainedby firing at 1,000 to 1,700° C., the rare earth oxide is distributed ata higher proportion in a portion of fine particles having a particlesize of up to 0.5 mm than in a portion of coarse particles having aparticle size in excess of 0.5 mm, and the crucible is substantiallyfree of the reaction products of the rare earth oxide with Al₂O₃ andAl₂TiO₅;

(6) a crucible for the melting of a rare earth alloy according to (4) or(5), characterized in that 2 to 100% by volume of the fine particleportion is the rare earth oxide;

(7) a crucible for the melting of a rare earth alloy according to (4),(5) or (6), characterized in that the at least one rare earth oxide isY₂O₃; and

(8) a rare earth alloy obtained using a crucible according to any one of(4) to (7).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmental cross-sectional view of a crucible according tothe invention in which a slag has generated.

FIG. 2 is a fragmental cross-sectional view of a conventional cruciblein which a slag has generated.

FIG. 3 is a fragmental cross-sectional view of another conventionalcrucible in which a slag has generated.

BEST MODE FOR CARRYING OUT THE INVENTION

A rare earth alloy melting crucible according to a first embodiment ofthe invention is one comprising Al₂O₃ and one or more oxides selectedfrom rare earth oxides inclusive of Y₂O₃ as main components,characterized in that the crucible is obtained by firing at 500 to1,800° C., the rare earth oxide is distributed at a higher proportion ina portion of fine particles having a particle size of up to 0.5 mm thanin a portion of coarse particles having a particle size in excess of 0.5mm, and the crucible is substantially free of the reaction product ofthe rare earth oxide with Al₂O₃.

The rare earth oxide which may be used is one or more oxides selectedfrom oxides of rare earth elements including yttrium (Y) and rangingfrom La to Lu. Of these, Y₂O₃, CeO₂, Dy₂O₃, Tb₄O₇ and Sm₂O₃ having alower degree of hydroxide formation are preferred for use.

The rare earth oxide is distributed in the fine particle portion with aparticle size of up to 0.5 mm in a high proportion in order to restrainreaction of the crucible with a rare earth alloy. Specifically, it ispreferred that at least 50% by volume, especially at least 60% by volumeof the rare earth oxides in the entire crucible be present in the fineparticle portion.

Also preferably, the content of rare earth oxide in the fine particleportion is 2 to 100% by volume, and especially 10 to 100% by volume.

In this portion, the balance may be one of ceramics such as Al₂O₃, SiO₂,TiO₂, ZrO₂, MgO, CaO, Si₃N₄, BN and TiB₂ or a combination thereof.

On the other hand, since the coarse particle portion having a particlesize in excess of 0.5 mm is relatively less reactive, it is possible touse Al₂O₃ having a good balance of mechanical strength, stability andcost. The content of Al₂O₃ is 20 to 100% by volume, and especially 50 to100% by volume of the coarse particle portion.

In this portion, the balance may be one of rare earth oxides such asY₂O₃, CeO₂, Dy₂O₃, Tb₄O₇ and Sm₂O₃ and ceramics such as SiO₂, TiO₂,ZrO₂, MgO, CaO, Si₃N₄, BN and TiB₂, or a combination thereof. Theaddition amount thereof is preferably 50% by volume or less. When therare earth oxide is used in the coarse particle portion, it must beincluded such that it is richer in the fine particle portion than in thecoarse particle portion.

It is noted that the size of coarse particles is preferably up to 10 mm,and especially up to 5 mm.

A rare earth alloy melting crucible according to a second embodiment ofthe invention is one comprising Al₂TiO₅ and at least one selected fromrare earth oxides inclusive of Y₂O₃ as main components, characterized inthat the crucible is obtained by firing at 1,000 to 1,700° C., the rareearth oxide is distributed at a higher proportion in a portion of fineparticles having a particle size of up to 0.5 mm than in a portion ofcoarse particles having a particle size in excess of 0.5 mm, and thecrucible is substantially free of the reaction product of the rare earthoxide with Al₂TiO₅; and

-   -   a rare earth alloy melting crucible according to a third        embodiment of the invention is one comprising Al₂O₃, Al₂TiO₅ and        at least one selected from rare earth oxides inclusive of Y₂O₃        as main components, characterized in that the crucible is        obtained by firing at 1,000 to 1,700° C., the rare earth oxide        is distributed at a higher proportion in a portion of fine        particles having a particle size of up to 0.5 mm than in a        portion of coarse particles having a particle size in excess of        0.5 mm, and the crucible is substantially free of the reaction        products of the rare earth oxide with Al₂O₃ and Al₂TiO₅.

The rare earth oxide which may be used is one or more oxides selectedfrom oxides of rare earth elements including yttrium (Y) and rangingfrom La to Lu, as in the first embodiment. Of these, Y₂O₃, CeO₂, Dy₂O₃,Tb₄O₇ and Sm₂O₃ having a lower degree of hydroxide formation arepreferred for use. Inter alia, Y₂O₃ having excellent stability is mostpreferred for use.

The rare earth oxide is distributed in the fine particle portion with aparticle size of up to 0.5 mm in a high proportion in order to restrainreaction of the crucible with a rare earth alloy. Specifically, it ispreferred that at least 50% by volume, especially at least 60% by volumeof the rare earth oxides in the entire crucible be present in the fineparticle portion.

Also preferably, the content of rare earth oxide in the fine particleportion is 2 to 100% by volume, and especially 10 to 100% by volume.

In this portion, the balance is preferably Al₂TiO₅, although it isacceptable from the standpoints of mechanical strength and the like toadd one of ceramics such as Al₂OB₂, SiO₂, TiO₂, ZrO₂, MgO, CaO, Si₃N₄,BN and TiB₂ or a combination thereof in an amount of up to 50% byvolume.

On the other hand, Al₂TiO₅ becomes the main component of the coarseparticle portion having a particle size in excess of 0.5 mm, andpreferably up to 5 mm.

Herein, part of Al₂TiO₅ may be replaced by Al₂O₃ which is commonly usedas the raw material of the crucible, although a mixture with a higherproportion of Al₂TiO₅ is preferred as long as mechanical strength isensured for the crucible.

When part of Al₂TiO₅ is replaced by Al₂O₃, the replacement proportion ispreferably up to 80% by volume and especially up to 30% by volume.

It is noted that to the coarse particle portion, one or a mixture of theabove-mentioned rare earth oxides and ceramics such as SiO₂, TiO₂, ZrO₂,MgO, CaO, Si₃N₄, BN and TiB₂, may be added in a proportion of up to 50%by volume. When the rare earth oxide is used in the coarse particleportion, it must be included such that it is richer in the fine particleportion than in the coarse particle portion, as previously described.

In the practice of the invention, the mixing proportion of fineparticles and coarse particles varies with the size of the crucible orthe like, and is preferably such that the fine particle portion is 10 to60% by volume, and especially 20 to 40% by volume. If the fine particleportion is less than 10% by volume, strength lowers with a highprobability. If the fine particle portion is more than 60% by volume, apossibility of failure by thermal shocks increases.

The crucible of the invention is produced, for example, by the followingprocedure.

Rare earth oxide and Al₂O₃ and/or Al₂TiO₅ are placed on a sieve havingopenings of a predetermined diameter (e.g., 5 mm or 0.5 mm), andclassified into a fine particle portion having a particle size of up to0.5 mm and a coarse particle portion (e.g., with a particle size of 0.5to 5 mm). The rare earth oxide is used for the fine particle portion,and Al₂O₃ and/or Al₂TiO₅ is used for the coarse particle portion.

The fine particle portion and the coarse particle portion are mixed sothat the fine particle portion accounts for 10 to 60% by volume. Aslurry of the mixture is filled in a predetermined mold where it isfired in an air atmosphere, vacuum atmosphere or inert gas atmosphere ofAr or the like, at 500 to 1,800° C., preferably 1,000 to 1,700° C.,obtaining a crucible.

If firing is effected at temperatures higher than 1,800° C., reactioncan occur between the rare earth oxide and Al₂O₃ and/or Al₂TiO₅, andsome areas are densified at the same time, with an increased possibilityof becoming brittle to thermal shocks. Additionally, the crucibledeforms due to the difference in shrinkage factor between reacted areasand unreacted regions, with the enhanced propensity of the cruciblebecoming unusable. On the other hand, heating at temperatures below 500°C. leads to under-firing, failing to achieve the desired strength.

Namely, by firing the crucible material in the above-defined temperaturerange, a crucible substantially free of the reaction products of therare earth oxide with Al₂O₃ and/or Al₂TiO₅ (and/or Al₂O₃) is obtainable.

As described above, the crucible for use in the melting of a rare earthalloy according to the invention uses Al₂O₃ and/or Al₂TiO₅ which is amaterial having a low coefficient of thermal expansion, for preventingcracks from generating upon thermal cycling, and has the rare earthoxide contained at a higher proportion in the fine particle portion witha particle size of up to 0.5 mm, for restraining the reaction of thecrucible with the rare earth alloy from proceeding.

Accordingly, as shown in FIG. 1, the reaction does not reach theinterior of a crucible 1 composed of coarse particle portions 10 andfine particle portions 11, with a slag 1A being weakly bound to thecrucible 1. Then the slag 1A can be readily removed, and any damage tothe crucible 1 during slag removal operation is avoided.

An additional advantage of an increased product yield arises from thereduced amount of slag, that is, the suppressed reaction of the cruciblewith the rare earth alloy.

Also contemplated herein is a rare earth alloy which is prepared usingthe inventive crucible described above.

The rare earth alloy is not critical as long as it contains one or morerare earth elements selected from among Y and La through Lu. Exemplaryalloys are Nd—Fe—B base alloys and Sm—Co base alloys.

Such rare earth alloy can be prepared by blending raw materials so as toprovide a predetermined composition, placing the blend in the inventivecrucible, effecting high-frequency melting in an inert gas atmospheresuch as Ar at 500 to 1,800° C., preferably 1,000 to 1,700° C., andpouring the resulting melt into a mold, followed by cooling.

It is noted that the temperature used in high-frequency melting ispreferably controlled within the above range in order to prevent anyreaction with the rare earth oxide, Al₂TiO₅ and other components of thecrucible.

EXAMPLE

Examples and Comparative Examples are given below by way of illustrationalthough the invention is not limited to these Examples.

Example 1

CeO₂ and Al₂O₃ were passed through two sieves of 5 mm and 0.5 mm andclassified into a fine particle portion having a particle size of up to0.5 mm and a coarse particle portion having a particle size of 0.5 to 5mm. CeO₂ was assigned to the fine particle portion, and Al₂O₃ wasassigned to the coarse particle portion.

They were mixed in a proportion of 30% by volume the fine particleportion and 70% by volume the coarse particle portion. A slurry of themixture was prepared and poured in a gypsum mold where it was allowed tostand over 2 days. By subsequent removal from the mold, holding for afurther 2 days and firing 1,550° C., a crucible was obtained. Thecrucible had an outer diameter of 540 mm, a height of 840 mm, athickness of 40 mm and a weight of 229 kg.

In this crucible was placed 500 kg of a raw material for Nd—Fe—B basemagnet which had been weighed so as to give the compositional formula:30.5 Nd-1.2 Dy-1.0 B-2.0 Co-0.2 Al-65.1 Fe (in % by weight). Aftermelting in an Ar atmosphere by high-frequency heating at 1,500° C. for70 minutes, the melt was cast into a mold.

The crucible was allowed to stand for 80 minutes for cooling and thenopened to air, followed by slag removal operation. Thereafter, the rawmaterial was placed therein and melted again. In this way, the processwas repeated until the crucible became unusable. The crucible failedduring the slag removal operation at the end of 58th melting cycle. Theproducts of these 58 cycles were in an average yield of 98.7%, and theaverage time taken for slag removal was 13 minutes.

Example 2

A crucible was prepared as in Example 1 except that a mixture consistingof 50% by volume of CeO₂, 30% by volume of Al₂O₃ and 20% by volume ofSiO₂ was prepared and used for the fine particle portion with a particlesize of up to 0.5 mm, and a mixture consisting of 10% by volume of CeO₂and 90% by volume of Al₂O₃ was prepared and used for the coarse particleportion with a particle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 54thmelting cycle. The products of these 54 cycles were in an average yieldof 98.5%, and the average time taken for slag removal was 15 minutes.

Example 3

A crucible was prepared as in Example 1 except that a mixture consistingof 80% by volume of CeO₂ and 20% by volume of Y₂O₃ was prepared and usedfor the fine particle portion with a particle size of up to 0.5 mm, anda mixture consisting of 70%by volume of Al₂O₃ and 30% by volume of SiO₂was prepared and used for the coarse particle portion with a particlesize in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 68thmelting cycle. The products of these 68 cycles were in an average yieldof 98.9%, and the average time taken for slag removal was 10 minutes.

Example 4

A crucible was prepared as in Example 1 except that Y₂O₃ was used forthe fine particle portion with a particle size of up to 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 75thmelting cycle. The products of these 75 cycles were in an average yieldof 99.0%, and the average time taken for slag removal was 9 minutes.

Example 5

A crucible was prepared as in Example 1 except that a mixture consistingof 50% by volume of Y₂O₃, 30% by volume of Al₂O₃ and 20% by volume ofSiO₂ was prepared and used for the fine particle portion with a particlesize of up to 0.5 mm, and a mixture consisting of 10% by volume of Y₂O₃and 90% by volume of Al₂O₃ was prepared and used for the coarse particleportion with a particle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 65thmelting cycle. The products of these 65 cycles were in an average yieldof 98.8%, and the average time taken for slag removal was 11 minutes.

Example 6

A crucible was prepared as in Example 1 except that a mixture consistingof 90% by volume of Y₂O₃ and 10% by volume of Dy₂O₃ was prepared andused for the fine particle port-ion with a particle size of up to 0.5mm, and a mixture consisting of 70%by volume of Al₂O₃ and 30% by volumeof SiO₂ was prepared and used for the coarse particle portion with aparticle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 72ndmelting cycle. The products of these 72 cycles were in an average yieldof 98.8%, and the average time taken for slag removal was 10 minutes.

Comparative Example 1

A crucible was prepared as in Example 1 except that Al₂O₃ was used forboth the fine particle portion with a particle size of up to 0.5 mm andthe coarse particle portion with a particle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 36thmelting cycle. The products of these 36 cycles were in an average yieldof 98.0%, and the average time taken for slag removal was 33 minutes.

Comparative Example 2

A crucible was prepared as in Example 1 except that a mixture consistingof 80% by volume of Al₂O₃ and 20% by volume of SiO₂ was prepared andused for the fine particle portion with a particle size of up to 0.5 mm,and a mixture consisting of 10% by volume of CeO₂ and 90% by volume ofAl₂O₃ was prepared and used for the coarse particle portion with aparticle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 33rdmelting cycle. The products of these 33 cycles were in an average yieldof 97.9%, and the average time taken for slag removal was 36 minutes.

Comparative Example 3

A crucible was prepared as in Example 1 except that a mixture consistingof 80% by volume of Al₂O₃ and 20% by volume of SiO₂ was prepared andused for the fine particle portion with a particle size of up to 0.5 mm,and a mixture consisting of 10% by volume of Y₂O₃ and 90% by volume ofAl₂O₃ was prepared and used for the coarse particle portion with aparticle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 37thmelting cycle. The products of these 37 cycles were in an average yieldof 98.1%, and the average time taken for slag removal was 29 minutes.

Comparative Example 4

A crucible was prepared as in Example 1 except that a mixture consistingof 99; % by volume of Al₂O₃ and 1% by volume of Y₂O₃ was prepared andused for the fine particle portion with a particle size of up to 0.5 mm,and a mixture consisting of 70%by volume of Al₂O₃ and 30% by volume ofSiO₂ was prepared and used for the coarse particle portion with aparticle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 1. Thecrucible failed during the slag removal operation at the end of 41stmelting cycle. The products of these 41 cycles were in an average yieldof 97.9%, and the average time taken for slag removal was 30 minutes.TABLE 1 Slag Fine particle portion Coarse particle removal (vol %)portion (vol %) Use Yield time CeO₂ Y₂O₃ Dy₂O₃ Al₂O₃ SiO₂ CeO₂ Y₂O₃Al₂O₃ SiO₂ cycles (%) (min) Example 1 100 — — — — — — 100 — 58 98.7 13Example 2 50 — — 30 20 10 — 90 — 54 98.5 15 Example 3 80 20 — — — — — 7030 68 98.9 10 Example 4 — 100 — — — — — 100 — 75 99.0 9 Example 5 — 50 —30 20 — 10 90 — 65 98.9 11 Example 6 — 90 10 — — — — 70 30 72 98.8 10Comparative — — — 100 — — — 100 — 36 98.0 33 Example 1 Comparative — — —80 20 10 — 90 — 33 97.9 36 Example 2 Comparative — — — 80 20 — 10 90 —37 98.1 29 Example 3 Comparative — 1 — 99 — — — 70 30 41 97.9 30 Example4

As shown in Table 1, a comparison of Examples 1-6 with ComparativeExamples 1-4 reveals that Examples containing rare earth oxide at ahigher proportion in the portion having a particle size of up to 0.5 mmsucceed in significantly increasing the use cycles of the crucible andare easy to remove slag and improved in product yield.

Example 7

Y₂O₃ and Al₂TiO₅ were passed through sieves of 5 mm and 0.5 mm andclassified into a fine particle portion having a particle size of up to0.5 mm and a coarse particle portion having a particle size of 0.5 to 5mm. Y₂O₃ was assigned to the fine particle portion, and Al₂TIO₅ wasassigned to the coarse particle portion.

They were mixed in a proportion of 50% by volume the fine particleportion and 50% by volume the coarse particle portion. A slurry of themixture was prepared and poured in a gypsum mold where it was allowed tostand over 2 days. By subsequent removal from the mold, holding for afurther 2 days and firing 1,550° C., a crucible was obtained. Thecrucible had an outer diameter of 540 mm, a height of 840 mm, athickness of 40 mm and a weight of 218 kg.

In this crucible was placed 500 kg of a raw material for Nd—Fe—B basemagnet which had been weighed so as to give the compositional formula:30.5 Nd-1.2 Dy-1.0 B-2.0 Co-0.2 Al-65.1 Fe (in % by weight). Aftermelting in an Ar atmosphere by high-frequency heating at 1,500° C. for70 minutes, the melt was cast into a mold.

The crucible was allowed to stand for 80 minutes for cooling and thenopened to air, followed by slag removal operation. Thereafter, the rawmaterial was placed therein and melted again. In this way, the processwas repeated until the crucible became unusable.

The crucible failed during the slag removal operation at the end of172nd melting cycle. The products of these 172 cycles were in an averageyield of 99.1%, and the average time taken for slag removal was 7minutes.

Example 8

A crucible was prepared as in Example 7 except that a mixture consistingof 50% by volume of Y₂O₃, 30% by volume of Al₂TiO₅ and 20% by volume ofSiO₂ was prepared and used for the fine particle portion with a particlesize of up to 0.5 mm, and a mixture consisting of 10% by volume of Y₂O₃and 90% by volume of Al₂TiO₅ was prepared and used for the coarseparticle portion with a particle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 7. Thecrucible failed during the slag removal operation at the end of 188thmelting cycle. The products of these 188 cycles were in an average yieldof 98.9%, and the average time taken for slag removal was 9 minutes.

Example 9

A crucible was prepared as in Example 7 except that a mixture consistingof 30% by volume of Y₂O₃, 30% by volume of Al₂TiO₅, 10% by volume ofAl₂O₃ and 30% by volume of SiO₂ was prepared and used for the fineparticle portion with a particle size of up to 0.5 mm, and a mixtureconsisting of 70%by volume of Al₂TiO₅, 20% by volume of Al₂O₃ and 10% byvolume of SiO₂ was prepared and used for the coarse particle portionwith a particle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 7. Thecrucible failed during the slag removal operation at the end of 196thmelting cycle. The products of these 196 cycles were in an average yieldof 98.8%, and the average time taken for slag removal was 11 minutes.

Comparative Example 5

A crucible was prepared as in Example 7 except that Al₂O₃ was used forboth the fine particle portion with a particle size of up to 0.5 mm andthe coarse particle portion with a particle size in excess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 7. Thecrucible failed during the slag removal operation at the end of 36thmelting cycle. The products of these 36 cycles were in an average yieldof 98.0%, and the average time taken for slag removal was 33 minutes.

Comparative Example 6

A crucible was prepared as in Example 7 except that Al₂O₃ was used forthe fine particle portion with a particle size of up to 0.5 mm andAl₂TiO₅ was used for the coarse particle portion with a particle size inexcess of 0.5 mm.

Using this crucible, a rare earth alloy was melted as in Example 7. Thecrucible failed during the slag removal operation at the end of 51stmelting cycle. The products of these 51 cycles were in an average yieldof 98.2%, and the average time taken for slag removal was 24 minutes.

The results of the foregoing Examples and Comparative Examples are shownin Table 2. TABLE 2 Slag Fine particle portion Coarse particle removal(vol %) portion (vol %) Use Yield time Y₂O₃ Al₂TiO₅ Al₂O₃ SiO₂ Y₂O₃Al₂TiO₅ Al₂O3 SiO₂ cycles (%) (min) Example 7 100 — — — — 100 — — 17299.1 7 Example 8 50 30 — 20 10 90 — — 188 98.9 9 Example 9 30 30 10 30 —70 20 10 196 98.8 11 Comparative — — 100 — — — 100 — 36 98.0 33 Example5 Comparative — — 100 — — 100 — — 51 98.2 24 Example 6

As shown in Table 2, a comparison of Examples 7-9 with ComparativeExamples 5-6 reveals that Examples containing rare earth oxide at ahigher proportion in the portion having a particle size of up to 0.5 mmsucceed in significantly increasing the use cycles of the crucible andare easy to remove slag and improved in product yield.

A comparison of Example 7 with Example 4 reveals that the life of thecrucible is significantly prolonged using Al₂TiO₅.

When Al₂O₃ or SiO₂ is added in order to increase the strength of thecrucible as in Examples 8 and 9, the use cycles are further increaseddespite some drops in product yield and slag removal time as comparedwith Example 7.

As described above, the present invention is successful in extending thelifetime of the crucible, reducing the slag removal time, increasing thethroughput of products, reducing the labor cost, and achieving anincrease in product yield.

1. A crucible for the melting of a rare earth alloy, comprising Al₂O₃and at least one selected from rare earth oxides inclusive of Y₂O₃ asmain components, characterized in that the crucible is obtained byfiring at 500 to 1,800° C., the rare earth oxide is distributed at ahigher proportion in a portion of fine particles having a particle sizeof up to 0.5 mm than in a portion of coarse particles having a particlesize in excess of 0.5 mm, and the crucible is substantially free of thereaction product of the rare earth oxide with Al₂O₃.
 2. A crucible forthe melting of a rare earth alloy according to claim 1, characterized inthat 2 to 100% by volume of the fine particle portion is the rare earthoxide and 20 to 100% by volume of the coarse particle portion is Al₂O₃.3. A rare earth alloy obtained using a crucible according to claim 1 or2.
 4. A crucible for the melting of a rare earth alloy, comprisingAl₂TiO₅ and at least one selected from rare earth oxides inclusive ofY₂O₃ as main components, characterized in that the crucible is obtainedby firing at 1,000 to 1,700° C., the rare earth oxide is distributed ata higher proportion in a portion of fine particles having a particlesize of up to 0.5 mm than in a portion of coarse particles ha size inexcess of 0.5 mm, and the crucible is substantially free of the reactionproduct of the rare earth oxide with Al₂TiO₅.
 5. A crucible for themelting of a rare earth alloy, comprising Al₂O₃, Al₂TiO₅ and at leastone selected from rare earth oxides inclusive of Y₂O₃ as maincomponents, characterized in that the crucible is obtained by firing at1,000 to 1,700° C., the rare earth oxide is distributed at a higherproportion in a portion of fine particles having a particle size of upto 0.5 mm than in a portion of coarse particles having a particle sizein excess of 0.5 mm, and the crucible is substantially free of thereaction products of the rare earth oxide with Al₂O₃ and Al₂TiO₅.
 6. Acrucible for the melting of a rare earth alloy according to claim 4 or5, characterized in that 2 to 100% by volume of the fine particleportion is the rare earth oxide.
 7. A crucible for the melting of a rareearth alloy according to claim 4 or 5, characterized in that the atleast one rare earth oxide is Y₂O₃.
 8. A rare earth alloy obtained usinga crucible according to claim 4 or 5.